Electrochemical energy storage involves a conversion between chemical and electrical energy and specifically includes chemical changes produced by electricity and the production of electricity by chemical changes. A chemical reaction on a product or products results in a new product or products. The difference standard Gibbs free energy can be used to determine the theoretical energy provided by a storage cell that uses a particular chemical reaction. If the chemical reaction is reversible or partially reversible (e.g., by applying an electric field to the cell), the cell can be considered rechargeable. Rechargeable cells allow for repeated discharge and charge cycles. The cycling behavior (e.g., storage capacity degradation during such cycling) can therefore be an important consideration.
Rechargeable batteries offer unique solutions to a growing number of energy and environmental issues. Many of these issues can be addressed by increasing battery energy storage efficiencies (e.g., in terms of capacity/mass). Accordingly, the development of rechargeable batteries with high specific energy (energy per mass) is an important step toward solving impending environmental and energy issues. Studies have hinted at the potential for the rechargeable batteries, which are believed to have less environmental impact than disposable batteries. Moreover, over their lifetime, rechargeable batteries can also use fewer natural resources and cost less. For many applications, the convenience of rechargeable batteries is paramount.
The basic premise for rechargeable batteries relies upon the transfer of charge between electrodes of the battery in one direction for charging and another direction for discharging. Charging of a battery involves movement of ions from the positive electrode to the negative electrode. This ionic movement is tied to electrical flow through a charge path of the battery (due to an applied voltage). Storage of energy occurs due to the ions combining with a material that makes up part of the negative electrode. When the battery discharges, the lithium ions move back to the positive electrode. This ionic movement is tied to the electrical flow through the discharge path of the battery (due to an applied load).
For all the promise of rechargeable batteries, there is room for improvement, and the ever-evolving technical landscape is generating demands on battery capabilities that are increasingly difficult to meet. One such demand relates to light batteries with high energy storage capabilities. Preferably, the high energy storage capacity should persist overtime and over multiple recharge cycles. Other demands relate to cost, safety, size or weight.
Li-ion batteries are believed to have the highest specific energy of all rechargeable batteries. Existing lithium-ion batteries are based on LiCoO2 cathodes and graphite anodes. This leading Li-ion battery technology is based on intercalation reactions and is believed to be limited to a theoretical specific energy of ˜370 Wh kg-1 for both the LiCoO2/graphite and LiFePO4/graphite systems. Notwithstanding, safety remains a major problem due to concerns arising from the formation of lithium dendrites during cycling, which can penetrate the separator and lead to thermal runaway. The separator can be implemented using a film-like material, made of electrically insulating polymer (e.g., polyolefin). The function of the separator is to inhibit electrons from flowing directly from anode to cathode so that they instead travel through an (external) discharge path. Penetration of such separators can lead to rapid discharge and overheating. Thus, separator penetration is to blame for thermal runaway events that sometimes result in explosive conditions.